1. Field of the Invention
The invention relates to the field of wireless digital communication using an apparatus and a method that achieves full diversity and low complexity without sacrificing bandwidth and with a linear complexity.
2. Description of the Prior Art
Antenna diversity, also known as space diversity, is any one of several wireless diversity schemes that use two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and distortions that can destructively interfere with one another at the aperture of the receiving antenna. Antenna diversity is especially effective at mitigating these multipath situations. This is because multiple antennas offer a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.
Inherently an antenna diversity scheme requires additional hardware and integration versus a single antenna system but due to the commonality of the signal paths a fair amount of circuitry can be shared. Also with the multiple signals there is a greater processing demand placed on the receiver, which can lead to tighter design requirements. Typically, however, signal reliability is paramount and using multiple antennas is an effective way to decrease the number of drop-outs and lost connections.
In the past, systems have been devised that have used time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) or other multiple access methods to avoid interference. The disadvantage is the waste of bandwidth resources.
Another way to avoid interference is to use antenna resources at the receiver to cancel the interference. The disadvantage of this method is that it reduces the diversity and/or increases the complexity exponentially.
Multi-user detection schemes with simple receiver structures have been recently well studied. Multiple transmit and receive antennas have been used to increase rate and improve the reliability of wireless systems. In this disclosure, we consider a multiple-antenna multi-access scenario where receive antennas are utilized to cancel the interference. In the prior art multiple antennas have been used to suppress the interference from other users. It has been shown that one can decode each user separately by using a large enough number of receive antennas. More specifically, for J users equipped with N transmit antennas, it is known how to cancel the interference using N J receive antennas.
To reduce the number of required receive antennas, the prior art has provided an interference cancellation method for users with two-transmit antennas. The method is based on the properties of orthogonal space-time block codes (OSTBCs) and requires a smaller number of receive antennas, i.e. as many as the number of users. This work was extended to a higher number of transmit antennas but only for J=2 users. The common theme of the prior art is the utilization of the properties of the orthogonal designs at the transmitter to cancel the interference at the receiver. In communications, multiple-access schemes are orthogonal when an ideal receiver can completely reject arbitrarily strong unwanted signals using different basis functions than the desired signal. One such scheme is time division multiple access (TDMA), where the orthogonal basis functions are non-overlapping rectangular pulses (“time slots”). Another scheme is orthogonal frequency-division multiplexing (OFDM), which refers to the use, by a single transmitter, of a set of frequency multiplexed signals with the exact minimum frequency spacing needed to make them orthogonal so that they do not interfere with each other.
Unfortunately, the method does not work for a general case of complex constellations, N>2 transmit antennas, and J>2 users. In fact, such an extension using orthogonal designs is impossible. Instead, it has been suggested that a method based on quasi-orthogonal spacetime block codes (QOSTBCs) might be used. The main complexity tradeoff between OSTBCs and QOSTBCs is the symbol-by-symbol decoding versus pairwise decoding. Therefore, by a moderate increase of decoding complexity, the prior art has extended prior multi-user detection schemes to any constellation, any number of users, and any number of transmit antennas.
Further, it is known that for M≧J receive antennas, the diversity of each user is equal to NM using maximum-likelihood detection and N (M−J+1) using low-complexity array-processing schemes. Note that the complexity of the maximum-likelihood detection increases exponentially as a function of the number of antennas, the number of users, and the bandwidth efficiency (measured in bits per channel use). Therefore, usually it is not practical.
The common goal and the main characteristics of the above multi-user systems are the small number of required receive antennas and the low complexity of the array-processing decoding. A receiver does not need more than J receive antennas and the decoding is symbol-by-symbol or pairwise using low complexity array-processing methods. One drawback, however, is that if we demand low complexity, the maximum diversity of NM is not achievable.